These studies are designed to investigate the regulation of gene expression following pharmacological treatments that alter the response to noxious stimulation of the periphery. We are particularly interested in gene regulation occuring in the dorsal root ganglia (DRG) which contain the cell bodies of primary afferent neurons, and in the spinal cord, where the first inhibitory modulation of nociception occurs. Two series of studies have been initiated. In the first series of experiments, using RNA blot analysis of the spinal cord, morphine treatment was found to inhibit the inflammation-induced increase in c-fos mRNA, prodynorphin and, to a lesser extent, proenkephalin mRNAs. These preliminary studies provide evidence that this methodology can be used to examine pharmacological effects on gene expression. The morphine- induced reduction of opioid mRNA expression after inflammation-induced hyperalgesia further support an action of morphine at spinal levels. The parallel decrease in c-fos mRNA provides additional support for a relationship between Fos transcription factor and opioid mRNA regulation. The second series of experiments addressed chronic morphine treatment and the induction of tolerance. Morphine often remains the only drug able to relieve persistent pain although the risk of tolerance is a major clinical concern. Thus, it is important to understand the mechanism of such phenomena. Our initial experiments looked at the effects of morphine and the morphine antagonist naltrexone on transcription factor mRNA regulation in the DRG. In rats treated intrathecally with morphine (30 ~g/kg/hr) for 4 days, RNA blot analysis identified an initial up- regulation in the expression of c-jun mRNA compared to saline-treated rats. Behaviorally, the animals were hypoalgesic to a radiant heat stimulus. In morphine tolerant rats, naltrexone administration produced withdrawal and led to an increase in c-jun mRNA expression in the DRG. These data, suggest that some genes responding to Jun transcription factor may have their expression regulated in response to long term morphine administration and withdrawal. Such changes could be part of the mechanism or the consequence of tolerance to morphine.